The use of so-called multi-mirror arrays (MMAs, also referred to as micromirror arrays or micromirror matrices), which include a multiplicity of individually drivable micromirrors, has recently been envisaged for illumination systems of microlithographic projection exposure apparatus in order to deviate individual sub-beams of the projection light of the illumination system in different directions. With the aid of the micromirrors, for example, the respective light sub-beams of the projection light can thus be directed onto different positions in a pupil surface of the illumination system. Since the intensity distribution in a pupil surface of the illumination system importantly influences the illumination angle distribution of the projection light, the illumination angle distribution can be specified more flexibly owing to the individual drivability of the micromirrors. Particularly in connection with so-called unconventional illumination settings, in which an annular region or a plurality of poles are illuminated in the pupil surface, the use of MMAs makes it possible to adapt the illumination angle distribution to the respective circumstances, and in particular to the mask to be projected, without for example diffractive optical elements having to be replaced.
Such MMAs are often produced as microelectromechanical systems (MEMS) via lithographic methods, such as are known from semiconductor technology. The typical structure sizes are sometimes a few micrometres. Known examples of such systems are, for example, MMAs whose micromirrors can be tilted digitally about an axis between two end positions. Such digital MMAs are often used in digital projectors for showing images or films.
For use in the illumination system of a microlithographic projection exposure apparatus, the micromirrors of the MMAs however are desirably capable of quasi-continuously adopting every tilt angle within a working angle range. In particular, the micromirrors are desirably tiltable about two tilt axes. Thus, for example, there are known micromirrors whose mirror surfaces are mounted in a type of universal suspension, and can be tilted with the aid of actuators, about two tilt axes. The actuators may, for example, be configured as electrostatic or electromagnetic actuators. In a known arrangement having electrostatic actuators, the micromirror with its two tilt axes is mounted over four control electrodes and has a mating or mirror electrode on its rear side. When a voltage is applied between a control electrode and the mating electrode, an attractive force action is created between them owing to electrostatic attraction. Each tilt axis is assigned two opposite control electrodes and the micromirror can be tilted in one direction or the other, depending on which electrode is driven. Through various combinations of tilting about the two tilt axes, the mirror can be tilted into any position within its working angle range.
In order to drive the individual actuators, each actuator has its own voltage or current supply line and the drive circuits connected therewith. In order to keep this outlay small and allow a maximally high packing density of the micromirrors in an MMA, US 2003/0189389 A1 therefore proposes a system in which only three actuators are used. Since the driving of the individual actuators no longer acts independently on a single tilt axis when using three actuators, however, a method is employed therein by which the three control signals of the actuators can be determined from two control variables for the tilting about the two tilt axes. In this method, the three control signals are calculated from the two control variables by using a linear equation system. The solution set of the equation system is restricted by various constraint conditions. For instance, one of the desired aspects of this conversion is that the two control variables respectively cause tilting about the associated tilt axes independently of one another. Another constraint condition is that the total force exerted by the three actuators onto the micromirror is kept constant.
Since the use of micromirrors in illumination systems of microlithographic projection exposure apparatus places very great demands on the accuracy of the mirror alignment, signal amplifiers such as for example high-voltage output stages, which supply the actuators with variable control signals, desirably also satisfy the most stringent of desired aspects with respect to signal stability and reproducibility in that system.